This invention relates to pulp screening apparatus, and more particularly to such apparatus including screens in which one or more rotating foils carried on arms, or other generally radial supports, move in relatively close proximity to a surface of the screen.
Rotating foils, often referred to as hydrofoils, are commonly employed in combination with pulp screening apparatus and move in relative rotation, in close proximity to the inlet surface of a screen plate. Such hydrofoils produce strong positive and negative impulses to the screen plates, which impulses tend to increase the screening capacity while keeping the screen openings clear and free of plugging. Such screen plates are often configured in the form of a cylindrical screen, and the hydrofoil is positioned to run along a radial inside or a radial outside surface of the perforated screen. An example of such a cylindrical screen plate is shown in the U.S. Pat. No. of Chupka et. al., 4,919,797 issued Apr. 24, 1990. However, hydrofoils are also used with non-cylindrical screens, such as conical screens or radially flat screens. An example of the latter is shown in the U.S. Pat. No. 5,078,275 (Chupka et. al.).
The hydrofoils themselves generally consist of a solid air-foil shaped body. When used with a cylindrical screen, the air foil body extends spanwise in a direction generally parallel to the axis of the cylinder. Often the hydrofoil body is formed with a lead angle or helical twist for the purpose of imparting some axial flow to the stock. Such a lead angle is also shown in U.S. Pat. No. 4,919,797 (Chupka et. al.).
While the hydrofoil in a pulp suspension (furnish) behaves differently than an airfoil or propeller section in air, it is convenient to use conventional air foil or propeller terminology in identifying like parts of hydrofoils. Thus, in a cylindrical screen embodiment, the radially outer curved surface which is positioned in closest proximity to the screen may be referred to as the camber surface. The opposite flat surface which faces the axis of rotation may be referred to as the face surface.
For any particular cylinder screen, furnish and a mode of operation, there is usually an optimum spacing, angle of attached and speed of the hydrofoil camber surface with respect to the adjacent surface of the screen for maximum reduction. The minimum available spacing may be defined by the presence of turbulence inducing bars or wear bars on the screen surface or by the profile or shape of the screen openings. As the gap between the role and plate is increased production decreases. Production increases as the gap decreases, so adjustment is critical to peak performance.
Spacing of the hydrofoils is also dictated by the screen roundness, or lack thereof. Out-of-round screens are common, particularly where the screen is fabricated from flat plate material which has been rolled into a cylinder shape and then welded. The actual running position of the foil must take into account any out-of-round characteristics of a particular cylinder, and it is not uncommon for 24-inch diameter cylinder to be out of round by as much as 0.090 inch.
Another factor which can influence the optimum spacing of the camber surface to the adjacent screen surface resides in the characteristic of the furnish being screened. With certain pulps, the suspension can be dirty. If the hydrofoil is positioned too close to the screen, it can cause the screen to wear at an accelerated rate. Further, too close a spacing can impart unnecessary and undue impulse stresses into the screen, which pulses can contribute to premature fracture or failure of such screens.
In cylindrical screening apparatus, adjustments of the radial positions of the hydrofoil has necessitated adjustment of the position of the support arms on the hub, and often has required removal of the screen cylinder from the screening apparatus in order to obtain access for the purpose of making such an adjustment.
Further, while many physical forms of hydrofoils have been suggested and tried, to the inventors' knowledge, such hydrofoils have not been formed bodies which have a separate section or element, movable with respect to the body, to vary the surface characteristics of the hydrofoil and/or to vary the gap or spacing at the throat between the hydrofoil and the screen surface.